git: show the version of `pygit2` with verbose version output
This seems like useful info to have when debugging. I followed the precedent of
hg-git, which prints something like:
hggit external 0.9.0a1 (dulwich 0.19.15)
We don't have a version number assigned (because it's internal), so it's just
the parenthetical.
Differential Revision: https://phab.mercurial-scm.org/D9436
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